Anionic contaminants in wastewater and other aqueous solutions pose significant environmental and health problems. For example, excessive levels of phosphate in seawater, freshwater, wastewater and sewage cause undesirable biological effects such as red tides and eutrophication. Arsenic in groundwater and mine wastewater threatens the health and lives of human beings, animals and plants when it is consumed. Therefore, there have been many efforts to remove these anionic contaminants from water.
With regard to phosphate removal, it is impossible to cost effectively remove this contaminant in cases of non-point source pollution, that is, in cases where the polluted water is drained from wide areas into rivers and the sea. Non-point source pollution streams are typically high volume streams with low concentrations of pollutants.
For point source pollution, where pollutants originate from specific points (e.g. industrial or domestic wastewater), current technologies include chemical precipitation, biological treatment, MBR (membrane Bio-coupled Reactor) method, ion exchange and absorption method.
The chemical precipitation method is widely used by small to medium sized sewage treatment plants to remove phosphate. However, at low concentrations, the efficiency of phosphate removal is low. Even at concentrations typical of domestic wastewater, removal efficiencies are typically less than 60 percent. In addition, this method generates a large amount of sludge, the disposal of which means extra cost.
Biological treatment is used by medium to large sized sewage treatment plants to remove phosphate. This method employs a biological pretreatment prior to the addition of coagulation/precipitation chemicals. It suffers from similar disadvantages as the chemical precipitation method discussed above.
The MBR method makes use of ultrafiltration membranes in a reactor design that allows for a continuous process as opposed to the batchwise precipitations in the methods described above. While this reactor technology is much more efficient in removing phosphate, it shares with the chemical precipitation and biological treatment technologies upon which it is based the disadvantage of producing concentrated sludge which must be removed from the reactor at intervals. It is also very expensive to install and maintain.
Ion exchange and reverse osmosis methods have been suggested as methods to remove arsenic in groundwater and mine wastewater. Semiconductor, electronics, and dyeing plants use ion exchange resins to remove a variety of charged contaminants. Because of the extremely high cost and limited capacity, wastewater pretreatment is required. This technology is appropriate only for specialized industrial purposes, and not for phosphate or arsenic removal in point/non-point source pollution. Reverse osmosis is similarly unsuited for such applications because it is very expensive and difficult to maintain.
For the foregoing reasons, there is a need for a means of removing anionic contaminants from aqueous solutions that is both highly efficient and cost-effective over a broad range of applications.